Mechanical waves are waves that require a medium. This means that they have to have some sort of matter to travel through. These waves travel when molecules in the medium collide with each other passing on energy. One example of a mechanical wave is sound. Sound can travel through air, water, or solids, but it can't travel through a vacuum. It needs the medium to help it travel. Other examples include water waves, seismic waves, and waves traveling through a spring.
Hi Kelly, I'm now 28 weeks pregnant with twins and I first started feeling them move when I was around 15/16 weeks pregnant but this is my second pregnancy. In my first pregnancy (not twins) I didn't feel any movement until I was 24 weeks pregnant. I think everyone feels their babies at different times so don't worry about anything.
Feeling your baby move for the first time in each pregnancy is called quickening. While a first time mother might not feel her baby move until after 18-22 weeks, as an experienced mother, you may notice that you feel your baby much sooner. You might notice quickening in a second pregnancy only slightly sooner, for example around 16-18 weeks. Or you may notice it a lot sooner, as in late first trimester. Your baby is moving from early pregnancy on. Though typically the baby is so small that it is difficult to feel.